High Tg fluorene-based hole-transporting materials for organic light-emitting diodes

Article abstract of DOI:10.1246/cl.2005.1604

A new series of fluorene-based hole-transporting materials, 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(diphenylamino)fluorene (1), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(di-p-tolylamino)fluorene (2), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(2-naphth

Full text of DOI:10.1246/cl.2005.1604

1604
Chemistry Letters Vol.34, No.12 (2005)
High Tg Fluorene-based Hole-transporting Materials for Organic Light-emitting Diodes
Ke-Feng Shao, Ying-Feng Li, Lian-Ming Yang, Xin-Jun Xu,y Gui Yu,y and Yun-Qi Liuy
Lab of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
yLab of Organic Solid, Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
(Received September 13, 2005; CL-051175)
A new series of fluorene-based hole-transporting materials,
aryldiamines determined by differential scanning calorimetry
(DSC) are summarized in Table 1. Obviously, Tg values (139–
171 ꢀC) of compounds 1–4 are higher than that of 1,4-bis(1-
naphthylphenylamino)biphenyl (NPB, Tg ¼ 100 ꢀC)11 widely
used as HTM in OLEDs. Meanwhile, TGA measurements for
1–4 revealed that no decomposition was found within 400 ꢀC.
The highest occupied molecular orbital (HOMO) energy
levels of 1–4 were determined by cyclic voltammetry (CV) in
a supporting electrolyte of 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) in THF using ferrocene as the inter-
nal standard. The HOMO–LUMO (lowest unoccupied molecular
orbital) band gap was calculated from the absorption edge of the
absorption spectrum of a thin film on quartz substrate. The
LUMO energy levels were calculated by subtraction of the
optical band gap from the HOMO energy levels. The results
are also listed in Table 1.
Double-layer device I–IV with a configuration of indium-
tin-oxide (ITO) (30 ꢀ)/triaryldiamines 1–4 (50 nm)/Alq3
(45 nm)/Al (100 nm), in which 1–4 were used as HTMs and
Alq3 used as electron-transporting material (ETM) and emitter.
A standard device with the same configuration using NPB as
HTM was also prepared under identical conditions for compar-
ison.
9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(diphenylamino)fluo-
rene (1), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(di-p-tolyl-
amino)fluorene (2), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis-
(2-naphthylphenylamino)fluorene (3), and 9,9-bis[4-(di-p-tolyl-
amino)phenyl]-2,7-bis(9-carbazolyl)fluorene (4), was prepared
and employed in the fabrication of organic light-emitting de-
vices (OLEDs). These materials were designed to have bulky tri-
arylamine moieties at the C-9 position, which provides them
with excellent thermal stability and high glass-transition temper-
atures (Tg ¼ 139{171 ꢀC). 1 and 3 are comparable to 1,4-bis(1-
naphthylphenylamino)biphenyl (NPB) in terms of device per-
formance (maximum luminence and current efficiency) in a
standard hole-transporting layer (HTL)/tris(8-hydroxyquino-
lino)aluminum (Alq3) double-layer device, but are superior to
NPB in terms of higher glass-transition temperatures (Tg ¼
139 and 145 ꢀC, respectively), while 2 shows higher luminance
and current efficiency as well as Tg (140 ꢀC) than those of
NPB (100 ꢀC).
Organic light-emitting diodes (OLEDs) have attracted much
attention because of their promising application in full-color flat-
panel displays in recent years1,2 since the initial work by Tang et
al.3 For practical applications, high efficiency and good durabil-
ity are particularly important among the device performances.
The morphological changes caused by the Joule heating
during the operation, especially in the hole-transporting layer
(HTL),4,5 seem to be one reason of degradation6,7 which reduce
the stability of OLEDs. Therefore, thermally and morphological-
ly stable organic hole-transporting materials (HTMs) are re-
quired to realize the practical application of OLEDs. Many
endeavors8–13 have been made to develop new HTMs with high
stabilities. For this reason, a new series of 9,9-bis(triarylamine)-
substituted fluorene-based triaryldiamines (1–4) with excellent
thermal and morphological stabilities have been synthesized
(Scheme 1),14 and the use of these triaryldiamines as HTMs in
OLEDs are studied.
Devices I–IV have almost the same emission peaks with the
NPB standard device from Alq3 (Figure 1), suggesting that the
charge recombination is localized in the Alq3 layer and triaryldi-
amines 1–4 act as hole transporter without causing exciplex for-
Table 1. Thermal and electrochemical properties of 1–4
Tg
Tm
Tc
EHOME ELUMO EGAP
/eV
Compound
/ꢀC /ꢀC /ꢀC
/eV
/eV
1
2
3
4
139 281 187
140 267 196
145 259 227
171 371 221
5.1
5.0
5.1
5.3
2.0
2.0
2.1
1.9
3.1
3.0
3.0
3.4
Glass-transition temperatures (Tg), melting temperatures
(Tm), and crystallization temperatures (Tc) concerning new tri-
1.0
I
II
III
IV
NPB
0.8
0.6
0.4
0.2
0.0
Ar1
N
Ar1
N
Ar2
Ar2
1: Ar1 = Ar2 = Ph
2: Ar1 = Ar2 = p-Tol
3: Ar1 = Ph, Ar2 = 2 -Np
4: Ar1, Ar2 = carbazolyl
N
N
300
400
500
600
700
800
Wavelength / nm
Figure 1. Normalized EL spectra of ITO/HTL/Alq3/Al de-
vices.
Scheme 1. Triaryldiamines 1–4.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.12 (2005)
1605
minance. Device I has a slightly higher brightness of 2180 cd/
m2 at 15.5 V than NPB standard device (1840 cd/m2, 15.5 V),
while device II has a luminance of 2870 cd/m2 at 15.5 V, which
is 55 percent higher than that of NPB device.
The current efficiency–current density characteristics
(Figure 2c) indicated that device I has a slightly higher current
efficiency (maximum, 1.17 cd/A, 290 cd/m2) than that of NPB
device (1.01 cd/A, 310 cd/m2), while device II was obviously
higher (1.59 cd/A, 270 cd/m2). This suggest that new triaryldi-
amine 1 is at least as good as NPB when used as HTM in OLEDs
but expected to be thermally more stable based on its higher Tg
value, while 2 has both higher performance in double-layer
HTL/Alq3 devices and better stability than NPB.
(a)
2800
I
II
III
IV
NPB
2400
2000
1600
1200
800
400
0
0
5
10
15
20
25
Voltage / V
In conclusion, these new materials were comparable to NPB
when used in OLEDs as HTMs (except for 4) in terms of
luminence and current efficiency, while 2 shows obviously high-
er luminance and current efficiency than NPB. Because of their
higher Tg than NPB, they are expected to be potential candidates
for application as HTMs in OLEDs. Since the device configura-
tion has not been optimized in the present work, further improve-
ment can be expected by fine-tuning of device structures.
(b)
I
II
III
IV
NPB
200
150
100
50
This work is financially supported by National Natural
Science Foundation of China No. 60276007.
0
0
5
10
15
20
Voltage / V
References and Notes
1
2
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-50
0
50
100 150 200
Current Density / mA cm-2
Figure 2. (a) Luminance–voltage, (b) current density–voltage,
and (c) current efficiency–current density characteristics of
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14 Synthesis and characterization data of these new compounds
are available from Supporting Information.
mation at the interface with Alq3. This could be attributed to the
proper HOMO energy levels of 1–4, which is favorable for hole
injection from ITO to HTL and then to the Alq3 layer.
Figure 2 shows the luminance–voltage (L–V), current–volt-
age (I–V) characteristics, and the current efficiency of the
devices. The current density and luminance of devices I and
III are comparable to those of NPB standard device, while devi-
ce II are superior to NPB standard device in terms of higher lu-
Published on the web (Advance View) October 29, 2005; DOI 10.1246/cl.2005.1604